Touch sensitive displays

ABSTRACT

Disclosed are a touch sensitive display and a method of operation thereof. The display comprises a sensor having an input put array of electrodes and, capacitively coupled thereto, an output array of electrodes and a controller operable to perform a scan operation at every intersection point of said input array. The scan operation comprises measuring a touch value for an intersection point; determining a proportional difference between said touch value and a base touch value for said intersection point as a proportion of said base touch value, wherein said base touch value is indicative of there being no touch event on the sensor; and comparing the proportional difference to a predetermined proportional touch threshold so as to determine whether there is a touch event at that point.

TECHNICAL FIELD

The present invention relates to arrangements for detecting user touchinput for use in touch sensitive displays and in particular fordetecting user multi-touch, i.e. more than one touch input from a userat the same time.

BACKGROUND

Personal computing devices equipped with touch sensitive displays arewell known and widely used. Such displays allow a user to control adevice by “touch inputs”, i.e. by touching a touch sensitive paneltypically positioned over a display screen.

Recent advances in so-called “multi-touch” technology have allowed thedevelopment of multi-touch devices, whereby a touch sensitive display ofdevice can derive control information from multiple simultaneous touchesby a user. Multi-touch technology increases the amount of control a userhas over a device and increases the usefulness and desirability of thedevice.

Development of multi-touch technology has been mainly limited tocomparatively small-scale personal computing devices such assmart-phones and tablet computers. However, there is recognition thatproviding multi-touch touch sensitive displays in other areas could leadto improved devices of other types.

Conventional multi-touch display devices use a so-called “mutualcapacitance” technique whereby the level of charge transferred from afirst set of conductors (i.e. electrodes) to a second set of conductorsby virtue of capacitive coupling is monitored. A reduction in thischarge transfer indicates a user touch. Other techniques can be used todetect user touch, such as so-called “self-capacitance” techniqueswhereby a change in capacitance of isolated conductors arranged in agrid pattern is monitored. However, self-capacitance based techniquesperform poorly when trying to distinguish between multiple simultaneoustouches and are therefore not appropriate for multi-touch applications.

A conventional mutual-capacitance based multi-touch display devicecomprises a touch-sensitive panel overlaid on a display screen. Thetouch sensitive panel includes a first (input) array layer comprising afirst set of conducting electrodes and a second (output) array layercomprising a second set of conducting electrodes. The first and secondarray layers are separated by a number of insulating layers andpositioned under a transparent protective substrate usually made fromglass. The first and second set electrodes are made from indium tinoxide (ITO). ITO when deposited in thin enough layers becomestransparent and is generally considered to be the best material for usein the panels that are positioned over display screens.

The electrodes of the first array layer are arranged to cross theelectrodes of the second array layer at a number of crossing points.Transfer of charge due to capacitive coupling between the electrodes ofthe first and second layers at the various crossing points is monitored.A user touch (e.g. a user bringing a finger or a capacitive stylus intoclose proximity or physical contact with the touch sensitive panel) isdetected when a drop in the level of charge transferred by capacitivecoupling is detected at a crossing point. This is due to charge thatwould otherwise have been transferred from one electrode layer to theother at the crossing point instead being transferred into the user (orstylus).

There are a number of drawbacks to conventional techniques for providingtouch sensitive panels for multi-touch devices. In particular, theprocessing of information from the output electrode array may be veryintensive, reducing responsiveness and deteriorating the userexperience. An example of this is the perceived requirement to repeatevery sensing scan at every crossing point in order to minimise theeffect of noise.

It would be desirable to reduce the processing overhead for touchsensitive displays.

In a first aspect of the invention there is provided a touch sensitivedisplay comprising a sensor having an input array of electrodes and,capacitively coupled thereto, an output array of electrodes; whereinsaid touch sensitive display comprises a controller operable to performa scan operation at every intersection point of said input array, saidscan operation comprising:

measuring a touch value for an intersection point;

determining the proportional difference between said touch value and abase touch value for said intersection point as a proportion of saidbase touch value, wherein said base touch value is indicative of therebeing no touch event on the sensor; and

comparing said proportional difference to a predetermined proportionaltouch threshold so as to determine whether there is a touch event atthat point.

Said controller may be further operable to measure said base touch valuefor each intersection point.

Said proportional touch threshold may be defined as a the differencebetween a touch value and a base touch value, as a proportion of saidbase touch value, indicative of there being a touch event.

Said predetermined proportional touch threshold may be set at any valuebetween 5% and 50%, or at a value between 10% and 30%.

Said sensor may be operable such that a change in voltage applied to oneof the electrodes of said input array results in a voltage pulse on theoutput array electrodes, the magnitude of which is indicative of a touchevent.

Each of the electrodes of said input array of electrodes and said outputarray of electrodes may comprise a conducting wire individuallyinsulated with an insulating coating. The conducting wire may comprise ametallic electrode material such as copper, nickel, tungsten or similar.The conducting wire may be of diameter 8 μm to 18 μm. The insulatingcoating may comprise a polyurethane coating. The insulating coating mayhave a thickness of 1 μm to 2 μm.

In some embodiments, the electrodes of said input array of electrodesare arranged substantially orthogonal to the electrodes of said outputarray of electrodes, each intersection point being where an inputelectrode crosses an output electrode.

Said input array of electrodes and said output array of electrodes maybe laid over each other so as to form a single electrode array layer inthe panel. In accordance with these embodiments the electrodes can belaid down on a supporting substrate as a single layer and in a singlemanufacturing step.

Said controller may be arranged to detect a touch at an intersectionpoint by transmitting a pulse on an input electrode and monitoring acorresponding pulse energy on one or more output electrodes, saidcorresponding pulse arising due to capacitive coupling between the inputelectrode and the one or more output electrodes. The touch may bedetected at the intersection point upon the controller unit detecting areduction in pulse energy on one of the output electrodes, compared tothe other output electrodes and/or to the same output electrode at anearlier time, said one of the output electrodes corresponding to theintersection point.

In a further aspect of the invention there is provided a method ofoperating a touch sensitive display, said touch sensitive displaycomprising a sensor having an input array of electrodes and,capacitively coupled thereto, an output array of electrodes; said methodcomprising performing a scan operation at every intersection point ofsaid input array, said scan operation comprising:

measuring a touch value for an intersection point;

determining the proportional difference between said touch value and abase touch value for said intersection point as a proportion of saidbase touch value, wherein said base touch value is indicative of therebeing no touch event on the sensor; and

comparing said proportional difference to a predetermined proportionaltouch threshold so as to determine whether there is a touch event atthat point.

Various further aspects and features of the invention are defined in theclaims.

Also described is a touch sensitive display comprising a sensor havingan input array of electrodes and, capacitively coupled thereto, anoutput array of electrodes; wherein said touch sensitive displaycomprises a controller operable to:

perform a scan operation at every intersection point of said input arrayand output array, so as to obtain a touch value for every point, saidtouch value being indicative as to whether there is a touch event atthat point, said intersection points being arranged into subsets;

compare said touch value to a predetermined first threshold; and

process only those subsets which comprise at least one intersectionpoint that has a touch value exceeding the first threshold.

For the avoidance of doubt it should be appreciated that a touch valueexceeding the threshold should not necessarily be taken to mean a touchvalue larger in magnitude. Where the touch value decreases from anuntouched “base” value as a result of a touch, exceeding the firstthreshold will comprise the touch value falling below (in magnitude) thefirst threshold.

Said intersection points may be arranged in rows and columns, whereinsaid subsets are either said rows or said columns. In a specificembodiment, the controller is further operable to determine and comparethe processing effort required in performing the processing step whensaid subsets are rows and in performing the processing step when saidsubsets are columns, and to select said subsets to be rows or columnsaccording to which requires the least processing effort. Saiddetermination may be based upon a determination of the number of rowshaving at least one intersection point that has a touch value exceedingthe first threshold, a determination of the number of columns having atleast one intersection point that has a touch value exceeding the firstthreshold, and the number of intersection points in each row and column.

Each of said subsets may have a counter attributed to it, said counterbeing incremented for each intersection point having a touch valueexceeding the first threshold comprised within its corresponding subset.Alternatively, each of said subsets may have a binary flag attributed toit, said binary flag being set should its corresponding subset comprisean intersection point having a touch value exceeding the firstthreshold.

Said predetermined first threshold may be at a level lower than that ofa touch threshold indicative of a touch event.

Also described is a touch sensitive display apparatus comprising:

a sensor comprising an input array of electrodes and, capacitivelycoupled thereto, an output array of electrodes;

a plurality of reception circuits, each reception circuit being operableto simultaneously receive a signal from a different electrode of saidoutput array of electrodes;

wherein said reception circuits each comprise an analogue to digitalconverter.

Each reception circuit may comprise a peak level detector and anamplifier.

Different subsets of said array of output electrodes may each beconnected to a different reception circuit, such that each receptioncircuit is arranged to receive inputs from a particular subset of saidarray of output electrodes. Said output array of electrodes may beevenly distributed into said subsets. Said output array of electrodesmay comprise parallel rows, said apparatus being arranged such that saidoutput array of electrodes are distributed into said subsets in aninterleaved manner, such that adjacent electrodes are not in the samesubset and not connected to the same reception circuit. In oneembodiment, where there are n reception circuits, each nth electrode ofsaid array of output electrodes, in sequence, may be connected to thesame reception circuit. “Row” in this context is not to be taken toimply a direction, and may mean vertical columns or rows arrangedhorizontally across the sensor.

Also described is a touch sensitive display comprising a sensor havingan input array of electrodes and, capacitively coupled thereto, anoutput array of electrodes; wherein said touch sensitive displaycomprises a controller operable to: perform a scan operation at everyintersection point of said input array and output array, so as to obtaina touch value for each intersection point, said touch value beingindicative as to whether there is a touch event at the intersectionpoint; and repeat said scan operation for only the intersection pointsfor which the touch value meets a predetermined criteria, no furtherscan operations being performed for the intersection points for whichthe touch value does not meet said predetermined criteria.

Said controller may be operable to compare said touch value to apredetermined resample threshold, wherein said predetermined criteriacomprises said touch value exceeding said predetermined resamplethreshold.

For the avoidance of doubt it should be appreciated that, a touch valueexceeding the threshold here should not necessarily be taken to mean atouch value larger in magnitude. Where the touch value decreases from anuntouched “base” value as a result of a touch, exceeding thepredetermined resample threshold will comprise the touch value fallingbelow (in magnitude) the predetermined resample threshold.

Said predetermined resample threshold may be at a level lower than of atouch threshold indicative of a touch event.

At each intersection point for which the scan operation is repeated, theresults of the repeated scan operations may be averaged. Between 1 and20 repeat scans may be performed for each of these intersection points.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, by reference to the accompanying drawings, in which:

FIG. 1 shows schematically a touch screen sensor suitable for use inembodiments of the invention;

FIG. 2 shows a specific implementation of a touch screen sensor suitablefor use in embodiments of the invention;

FIGS. 3 a and 3 b are schematic diagrams illustrating the basicoperating principle of mutual capacitance PCT sensors;

FIG. 4 shows schematically a touch sensitive device apparatus accordingto embodiments of the invention;

FIG. 5 shows schematically a detail of the receive circuit of FIG. 4;

FIG. 6 is a flowchart illustrating an improved resampling method;

FIG. 7 shows an example screen output, illustrating the performancegains of the method illustrated in FIG. 6;

FIG. 8 is a flowchart illustrating a method for detecting a touch;

FIGS. 9 a and 9 b are flowcharts illustrating an improved method fordetecting a touch;

FIG. 10 shows an example screen output, illustrating the performancegains of the method illustrated in FIGS. 9 a and 9 b;

FIG. 11 shows schematically a touch sensitive device apparatus havingparallel receive circuits;

FIG. 12 illustrates an output electrode arrangement; and

FIG. 13 illustrates an improved interleaved output electrodearrangement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Touch screens are well known in the art. They work by sensing thepresence of a touching object on the screen surface. There are a numberof different touch screen implementations, each having their ownadvantages and disadvantages. Some touch screens, known as multi-touchscreens, are able to resolve the positions of two or more simultaneoustouches on the screen.

Capacitive touch screens rely on a conductive object (such as a finger)affecting the capacitance locally at a screen surface. Differenttechnologies may be used to determine the location of the touch. Thelocation is then sent to a controller for processing. Mutual CapacitanceProjected Capacitive Technology (PCT) is a variant of capacitive touchtechnology. Applying a voltage to a grid of electrodes creates anelectrostatic field, which can be measured. When a conductive objectcomes into contact with a PCT panel, it distorts the local electrostaticfield at that point, resulting in a measurable change in capacitance.The capacitance can be changed and measured at every individual point onthe grid.

FIG. 1 shows a touch screen sensor 100 such as that used in embodimentsof the invention. It comprises input electrodes (column or x-electrodes)120 and output electrodes (row or y-electrodes) 110. At every row/columnintersection or crossing point there is no direct electrical connection,as the input and output electrodes are electrically isolated. Instead acapacitor is formed at the intersection due to the proximity of theinput and output electrodes at that point. The electrodes 110, 120 maybe layered on a sheet of glass. This can be done, for example, byetching column and row electrodes separately on two perpendicular layersof conductive material, with parallel lines or tracks to form a grid.Such arrangements may use an Indium Tin Oxide (ITO) coating to createthe electrodes, which are insulated from one another.

FIG. 2 illustrates a novel alternative to this arrangement, and isdescribed in detail in a number of other patent applications being filedby the present applicant on the same day as this application. In thisarrangement, the electrodes 200 comprise arrays of insulated conductingwires. The electrodes 200 are attached to a front glass substrate 210 bya layer of adhesive 220. The layer of electrodes 200 are protected ontheir other side by a protective layer 230 which may comprise PET film.

The electrodes 200 may comprise copper wire of, for example, 8 μm to 18μm thickness. The insulation may comprise 1 μm to 2 μm thickpolyurethane insulation. This allows the input and output electrodes 200to be deposited over each other on the same layer without shorting witheach other. Since the input and output electrodes 200 can be depositedon a single layer, a simplified multi touch sensor construction can beachieved over current traditional ITO sensor constructions.

FIGS. 3 a and 3 b illustrate the basic principle of how mutualcapacitance PCT sensors operate. It shows an input electrode 300 andoutput electrode 310 which form part of an input array (or x-array) andoutput array (or y-array) respectively. A pulse of energy is transmittedto the input electrode 300 and, at intersections of each input andoutput electrode, is received via capacitive coupling (represented byelectric field lines 320) by the output electrode 310. Electroniccircuitry (represented by amplifier 330) measures the energy received inthe output array, symbolically represented here by meter 340 a. FIG. 3 bshow what happens when a conductive object (here a finger 350) isapplied to the sensor. The applied finger 350 capacitively couples someof the energy 320 away to ground, with any residual energy beingreceived by the output array of electrodes. Consequently meter 340 b isshown with a lower reading than meter 340 a. This drop in energy ismeasurable, and its location can be determined for each touch, evenwhere there are multiple simultaneous touches.

FIG. 4 shows schematically a touch sensitive apparatus comprising asensor 400. The sensor, as before, comprises an input array ofelectrodes 420 and an output array of electrodes 410. Typically a sensormight have 80 input electrodes and 48 output electrodes. A levelgeneration circuit 430 is used, via multiplexer 440, to rapidly increasethe voltage on each of the input electrodes in turn. As a result, asmall pulse appears on each of the output electrodes, the size of thepulse being dependent on the extent of the capacitive coupling.Typically for a +24V swing on one of the input electrodes, each outputelectrode receives a (order of) 50 mV pulse. This is reduced by about athird if a finger is in contact with the sensor glass near to theparticular intersection point. Pulses on each output electrode arereceived in turn, via multiplexer 450, by a receive circuit 460 and thenmeasured and processed by microprocessor 470. Microprocessor 470 maycontrol some or all aspects of this touch sensitive apparatus, includingthe calculations required for touch detection.

FIG. 5 shows a possible implementation of the receive circuit 460 ofFIG. 4. The circuit comprises amplifier 500, peak detector input switch510, a peak detector 515 comprising diode 520 and capacitor 530, a resetswitch 540 and an analogue to digital converter (ADC) 550, arranged asshown.

Multiplexer 450 sequentially connects receive circuit 460 to each outputelectrode of the output array 410. In this way, a pulse signal V_(in),is received by the receive circuit 460. The operational amplifier 500amplifies the signal V_(in), to (for example) an approximately 1.5Vpulse. Peak detector input switch 510 enables this signal to bepresented to peak detector 515. The peak detector 515 identifies themaximum level of the signal. Peak detector input switch 510 thendisconnects the incoming level so that the peak detector 515 maintainsthis peak value. An ADC 550 samples the level stored on the peakdetector 515. Finally a reset switch 540 is operated to short the peakdetector capacitor 530 and return the level on the peak detector tozero.

Use of a peak detector 515 is beneficial because the timing of the pulseis not critical. However, a simple “sample and hold” may be used instead(for example by removing the diode 520 from the circuit). The twomechanisms have different properties for noise rejection and EMCperformance.

Resample on Touch

A complete sensor scan consists of a number of hardware scan operations,each evaluating the capacitive coupling between one input electrode andone output electrode. The number of hardware scan operations required toscan the whole sensor once is:

N_(x)*N_(y)

where N_(x) is the number of input electrodes and N_(y) is the number ofoutput electrodes.

However the measurements comprise some level of noise. Minimising noiseis important for various reasons, such as avoiding false touches andgetting good touch location precision. As a consequence, multiple scanoperations should be performed at each intersection, for each completesensor scan operation, with the results combined (for example) byaveraging them. A typical number of hardware scans performed andaveraged in each complete sensor scan may be three. Consequently, thenumber of hardware scan operations required for a complete sensor scanmay be:

N_(x)*N_(y)*3

This takes much longer than a single scan, and results in a drop in theframe rate. This ultimately makes the user's experience of the touchoperation to feel slower, or to perform less well. For example, theremay be a reduced number of points when drawing a circle, making thecircle appear rather staggered.

It is therefore proposed to perform multiple sampling only at the pointsat or near a potential touch. This “resample on touch” operationdelivers the advantages of reduced noise whilst avoiding much of theadditional time overhead. FIG. 6 is a flowchart describing thisoperation. At step 600, a new intersection is considered and, at step610, scanned. The measurement from this scan is compared to a thresholdat step 620. Only if the scan measurement is above the threshold is thepoint re-sampled and averaged (step 630). A further two scans may beperformed during this step, for example. However, if the comparison atstep 620 is below the threshold, the re-sample step 630 is omitted. Atstep 640 it is determined whether all intersections have been scanned:if yes, the scan stops 650; if no, the scan returns to step 600 and theroutine repeated for the next intersection.

By using this method the total number of hardware scan operationsbecomes:

(N_(x)*N_(y)*1)+(N_(T)*s)

where N_(T) is the total number of points measured above the thresholdand s is the number of additional scans performed at these points (whichmay be two, for example).

A typical reduction in processing effort can be seen from the followingexample shown in FIG. 7. Its shows a screen having 16×16 cells. Thenumber shown for each cell is a relative scan measurement result(arbitrary units). In this example, the resample on touch thresholdlevel has been set to be the same as the touch threshold level, althoughthey can be different. Purely for illustration, the threshold level hereis 10, such that a touch is detected and resampling performed for allcells measure over 10. A normal complete scan for this screen would take16*16*3=768 operations.

The screen is shown with four points having been touched. The darkestcells are those measured above the touch/resample threshold level (onetouch may result in more than one adjacent cell being measured above thetouch threshold). Here there are seven such cells. Therefore, using themethod of FIG. 6 to perform a complete scan, and taking three samples ateach location measured above the threshold, results in a total number ofhardware scan operations:

16*16*1+7*(3−1)=270

As mentioned previously, the resample threshold level may be set lowerthan the touch threshold level. Using the example of FIG. 7, with theresample level illustratively set to 5 (cells shown with lightershading), it can be seen that only an additional 13 cells needresampling. This results in a total of 296 hardware operations for acomplete scan.

Good quality information is still available at all touched locations,which can be used to make good location estimates. The increased speedcomes as a result of not wasting effort in gathering reduced noiseinformation at locations that are not touched. In the specific exampleshown, the number of hardware scan operations required has been reducedfrom 768 to 270, or 65%. This saving can be used to save energy orreduce hardware resources or processing effort, or to increase the framescanning rate.

The above example is for four touches on a 16×16 grid. The ratio ofspeed up varies with number and size of touches, the sensor size,threshold limits and number of resamples. Larger screens, for example,will not only have a larger grid, but usually require more resampling toreduce noise sufficiently (say five times). Consequently, greaterefficiency savings can be realised for these screens.

Row/Column Elimination

In order to detect touches, a region detector is provided whichidentifies individual contacts from an array of scan data. It does thisby identifying local peaks in the scan data. FIG. 8 illustrates how thismay be done. For each intersection (step 800), it is determined whetherthe measured value is above a threshold (step 810). If it is above athreshold then the measured value is compared to each surrounding valuein turn (step 820). If the measured value is higher than that of thesurrounding values it is flagged as a peak (830). This process isrepeated for each intersection until complete (steps 840 and 850). Thismethod is somewhat intensive and takes a large amount of processor time,even on a fast embedded processor.

FIGS. 9 a and 9 b illustrate an improved process which reduces thisprocessing time and effort. The process operates in two parts. FIG. 9 aillustrates the first of these parts, which is performed in the scanner,that is the apparatus which actually performs the scanning of the outputelectrodes to detect touch. The process uses two counter value arrays, aRow Counter array RC, for counting the number of rows (each row may bedefined by an output electrode) for which a touch is detected (orthreshold exceeded), and a Column Counter array CC, for counting thenumber of columns (each column may be defined by an input electrode) forwhich a touch is detected (or threshold exceeded).

At step 900, arrays RC and CC are reset. At step 905, as each value isscanned and stored, it is compared to a threshold level 910. (Thiscomparison may be the same as that performed for the previous “resampleon touch” method, and therefore this step may be shared if both of thesemethods are being performed). If the value is above the threshold, a rowcount array RC counter, and a column count array CC counter are bothincremented for the corresponding row and column (step 915). Thisprocess is repeated for each intersection until complete (steps 920 and925).

FIG. 9 b illustrates the second part of this process, performed in theregion detector (that is the apparatus which processes the scanneroutput to calculate which region of the sensor that a touch was made).At step 930 the number of RC counter values above zero in the row countarray RC are counted. This represents the number of rows in which avalue above the threshold can be found. At step 935, the number of CCcounter values above zero in the column count array CC are counted. Thisrepresents the number of columns in which a value above the thresholdcan be found. At step 940 it is determined which of these counter arrayshave a greater number of values above zero. If it is the column countarray CC that has the greater number of non-zero values then, at step945 a first row is selected for which (at step 950) it is determinedwhether the row count array RC counter is above zero. If it is abovezero, each value in the row is processed one by one (step 955),otherwise the row is skipped. This is repeated for each row. If thedetermination at step 940 goes the opposite way, then, at step 960 afirst column is selected for which (at step 965) it is determinedwhether the column count array CC counter is above zero. If it is abovezero, each value in the column is processed one by one (step 970),otherwise the column is skipped. This is repeated for each column.

A typical reduction in processing effort can be seen from the example ofFIG. 10. Here four points on the screen have been touched. The darkestshaded cells are above the touch threshold level, which here is set to10. The scanner also provides a row count array RC and column countarray CC (bold digits). These contain the number of values abovethreshold in each row and column respectively. For example, the secondvalue in the row count array RC is set to 2 because two values in thatrow (11 and 20) are above the threshold level. This is therefore a“non-zero” row. In contrast the first value in the row count array RC isset to zero as no values in that row are above the threshold level: itis a “zero-row”. Note that the touch threshold and threshold forincrementing the row or column counter is the same in this example,although this does not have to be the case.

Some inputs (e.g.: four fingers along the same row) favour “zero row”elimination, whilst others (e.g.: four fingers along the same column)favour “zero column” elimination, and the algorithm attempts to selectthe optimal basis for elimination based on the data in the presentframe. In the specific example shown here, the region detector countsthe number of non-zero rows: here there are 4. It then counts the numberof non-zero columns: here there are 5. As there are fewer non-zero rowsthan non-zero columns, the region detector decides to go through thedata row by row. Of course, for non-square sensors (that is sensors forwhich the number of columns and number of rows are not equal), decidingwhether “zero row” elimination or “zero column” elimination is preferredis not as straightforward as shown here. A different criteria should beselected which takes into account the relative difference in the row andcolumn sizes when determining the most efficient method. Formulatingsuch criteria, and therefore adapting the method of FIG. 9 baccordingly, is well within the capabilities of the skilled person.

By examining the row count array RC in this specific example, it can beseen that the first row can be skipped with no further effort. The nexttwo rows are processed, the next two skipped, the next two processed,and then the entire lower half of the sensor is skipped. This results invery significant processing effort reduction.

In the example shown in FIG. 10, the number of items the region detectorhas to process using the method of FIG. 8 is:

16*16=256

Using the method of FIGS. 9 a and 9 b, the number of items the regiondetector has to process (in this specific example) is:

16*4=64

which is a processing reduction of 75%. The improvement varies fordifferent sensor sizes, number of touches, threshold limits and touchpatterns, for example, with just a single touch even better performancegains will be observed. In most cases a substantial processing timeimprovement will be achieved, which can be used to improve frame ratesor reduce the cost of processing hardware.

It should be noted that a special case exists where there are notouches, as the region detector avoids examining any data values at all.

Other mechanisms can also see efficiency improvements from row/columnelimination in a similar manner. For example blur and despeckle filters,and other parts of the region detector, may benefit from these methods.

The specific row/column elimination method illustrated here may bevaried. For example, an alternative to having integer row count andcolumn count arrays, it would be possible to have binary flags(indicating either “zero-row/column” or non-zero row/column) and simplyset these when a value in its corresponding row/column is above thethreshold. Also, the threshold for incrementing the array counter orsetting the binary flag may be different to the touch threshold. Usingthe FIG. 10 example, with this threshold set at 5, only an additionaltwo rows (the first and fourth row) need to be processed.

Note that row and column elimination can be used together as in thisexample, or only one of row elimination or column elimination could beused on its own. Using them together should be expected to giveadvantages in a greater number of situations, at the expense of slightlymore code in order to handle both.

Parallel Receive and Convert

As explained in relation to FIGS. 4 and 5, a full sensor scan iscompleted by repeatedly applying changes to the voltage on inputelectrodes on one axis (x) whilst monitoring the size of the voltagespike induced on electrodes on the output electrodes of the other axis(y). The voltage on the output electrode is amplified and peak detected,and this level is measured with an ADC (analogue to digital convertor).

When using a single pulsing circuit, a single receive circuit and asingle ADC, a simple estimate of the time taken to do a full sensor scancan be calculated as:

Scan time=ECX*ECY*(t _(p) +t _(ADC))

where ECX is the number of input sensor electrodes, ECY is the number ofoutput sensor electrodes, t_(p) is the pulse time+t_(ADC) is the timefor ADC reading.

Using the following example parameters in the above scan time equation:ECX 32 80, ECY=48, Pulse time=1.0 μs (estimated, depends on settlingtime desired for the previous signal to decay between pulses) and timefor ADC reading=1.0 μs (estimated, depends on ADC speed and accuracysettings used); the scan time will be 80*48*(1.0 μs+1.0 μs)=7.68 ms.Therefore the scan rate, which is the reciprocal of the scan time, is130 Hz.

The scan rate is very important in practice, as high scan rates meangood touch response for the user. Also under noisy conditions it ispossible to reduce the effect of noise by repeating scans, which isclearly more feasible when the scan can be performed quickly.

A possible way of improving the scan time may be to provide multiplereceive circuits (one for each output electrode), multiplexed into asingle ADC. This would only require a single pulse to capture responsevalues at each point down an input electrode column. The ADC is thenused to measure these captured values one at a time. This is repeatedfor each column. The total sampling time for this configuration can beestimated as follows:

Scan time=ECX*(1*(t _(p))+ECY*(t _(ADC))).

Using the values above the scan time may be 80*(1*1.0 us+48*1.0 us)=3.92ms, in which case the scan rate would be 255 Hz. This is a substantialspeed up over a system with only a single receive circuit.

FIG. 11 shows an arrangement which can further improve on this scanrate, while reducing the hardware overhead. The arrangement is similarto that of FIG. 4, but instead of receive circuit 460, there are threereceive circuits 1160 a, 1160 b, 1160 c. Each of the receive circuits1160 a, 1160 b, 1160 c may be the same as that shown in FIG. 5. Ofparticular importance is that each receive circuit has its own ADC. Eachreceive circuit 1160 a, 1160 b, 1160 c is connected to the output arrayvia a dedicated multiplexer 1150 a, 1150 b, 1150 c, with the electrodesof the output array being shared between the multiplexers 1150 a, 1150b, 1150 c.

By using three receive circuits, rather than 48 as in the previousexample, much less hardware is required (reducing cost and board space),but by using 3 ADCs it is still possible to complete scans very quickly.For each transmitted pulse the receive pulse is measured on three outputelectrodes at once.

The total sampling time for this configuration is as follows:

Scan time=ECX*(ECY/3)*(t _(p) +t _(ADC))

Using the example values above the scan time can be estimated to be80*(48/3)*(1.0 us+1.0 us)=2.56 ms, providing for a scan rate of 391 Hz.This is a substantial speed up over previously known configurations,despite having reduced hardware costs.

As mentioned in relation to FIG. 5, while peak detect operation is usedhere, a timing based mechanism without a peak detector could be made towork as well. Also, in practice a further improvement in scan rate canbe achieved by arranging for the pulse dying away time to occur whilstthe ADCs are measuring the peak voltage.

It should be noted that, while three receive circuits have beendetermined to provide a particularly beneficial compromise between scantime and hardware overheads, the arrangement of FIG. 11 may be amendedto comprise a different number of receive circuits, for example 2, 4 or5.

Receive Circuit Interleaving

It is proposed above to provide for parallel receive circuits to enableparallel reading of a number of the receive electrodes. In one example,three such receive circuits are provided, allowing three receiveelectrodes to be simultaneously read. The examples below show howinterleaving the multiplexor receive circuit can reduce scan time onsmaller sensor sizes with a reduced number of receive electrodes withoutmodifying the hardware structure.

In a normal sized array there may be 48 (or another multiple of three)receive electrodes. In such a case there will be 16 scan periods, withthree electrodes simultaneously scanned in each scan period. The receivecircuits are usually arranged such that one receive circuit is connectedto a first group comprising the first (e.g. top) 16 receive electrodesin the array, another receive circuit is connected to a second groupcomprising the next (e.g. middle) 16 electrodes in the array and theother receive circuit is connected to a third group comprising the last(e.g. bottom 16 electrodes in the array. In each scan period, oneelectrode from each group is scanned, such that the first electrodes ineach of the first, second and third groups are scanned in the first scanperiod, the second electrodes in each of the first, second and thirdgroups are scanned in the second scan period, and so forth.

FIG. 12 illustrates the drawback with such a receive arrangement when asmaller sensor (that is a sensor with fewer receive electrodes) isconnected to it. Shown here is an example of a sensor with 34 receiveelectrodes. As before they are arranged in groups, so that the firstgroup comprising the top 16 electrodes (A1-A16) are connected to onereceive circuit, and the second group comprising the middle 16electrodes (B1-B16) are connected to another receive circuit. This meansthat one receive circuit is connected to only two electrodes (C1-C2)which make up the third group. This is inefficient and results in thesame number of scan periods being needed to scan this smaller 34electrode array as was needed to scan the 48 electrode array. In onlythe first and second scan periods are three electrodes scannedsimultaneously: A1, B1 and C1 in the first scan period and A2, B2 and C2in the second scan period. In each of the following scan periods onlyelectrodes from the first and second groups are scanned.

FIG. 13 shows how the electrodes may be interleaved between the receivecircuits, such that each successive electrode is connected to adifferent one of the receive circuits in turn. Consequently the firstgroup of electrodes (A1, A2, A3 etc.) comprise every third electrode,starting from the first, the second group of electrodes (B1, B2, B3etc.) comprises every third electrode starting from the second electrodeand the third group of electrodes (C1, C2, C3 etc.) comprises everythird electrode starting from the third electrode. In this way, when asensor comprising 48 receive electrodes is being read, 16 scan periodsare still required, but when a sensor of (say) 34 electrodes isconnected, only 12 scan periods will be needed and efficiency savingscan be achieved.

Percentage Scanner

As previously mentioned, when a large (e.g. 24V) step is applied to oneof the input electrodes, a small pulse is induced on each of the outputelectrodes. Typically, with the sensor untouched, this is about 50 mV.This pulse is processed by the receive circuit, during which the smallpulse is amplified by the receive circuit's amplifier. If, for example,the amplifier has an effective gain of 50, and the peak detector diodedrops a fixed 0.5V, the resulting peak level detected and converted bythe ADC may be 2.0V.

The actual magnitude of the induced pulse depends on whether or notthere is an object touching the point where the pulsed input electrodecrosses the output electrode being measured. Typically this reduces theheight of the pulse seen on the output electrode by up to about 40%.Therefore, using the same example values, when touched, an outputelectrode might typically see a pulse of about 30 mV. This is amplifiedto about 50*30 mV=1.5V. The peak detector diode drops 0.5V resulting intypically 1V measured at the ADC.

In order to detect a touch, a threshold level of 1.5 volts may be set.The level of the threshold is important for distinguishing real touchesfrom noise. The voltage reading is also important for estimating theexact location of a touch when it is somewhere between two electrodes.The interpolator looks not just at the electrode for which the signal ismost affected by the touch, but also at the levels of the electrodesaround it. From this it is able to estimate the exact x, y position of atouch to much greater precision than “nearest electrode”. In theory,where a sensor has only 80×48 electrodes, the co-ordinate position ofthe touch can be resolved to a point on a (for example) 4096×4096 grid(although due to signal noise the result will never be exact). Measuringvery precisely the amount that a signal is reduced by is important forgetting interpolator operation to work well.

Unfortunately, electrode voltage levels do not all start the same. Withan untouched sensor it can be observed that the voltage peaks varysomewhat depending on which input and output electrodes are respectivelypulsed and measured. This can be for many reasons, such as proximity toa metal frame, or track conductivity under certain circumstances. Oftenthe most severe difference is seen at the extreme edge electrodes, whichcan see a pulse on the output electrode that is significantly reduced,perhaps (for example) to about a half of the level seen on otherelectrodes.

If this were the case, after processing by the receive circuit, the peaklevel detected may be 0.75V. With a threshold level set at 1.5 volts,this edge electrode reading would always be detected as a touch, evenwhen untouched.

A possible method to address this issue is to make a scan of voltagelevels when the sensor is untouched. This could be recorded as a full x,y grid of offset values, one for each intersection point on the sensor.In practice, readings may be repeated several times and averaged toreduce the effect of noise. The resultant values can be subtracted, inoperation, from the offset values. A difference value of 0 indicates notouch. A large difference value indicates a touch.

For example, in the middle of the sensor the following values may beseen:

-   -   Untouched “offset” value=2.0V    -   Touched value=1.0V    -   Difference=1.0V

Consequently a difference threshold of 0.5V may be chosen, such thatanything with a difference of 0.5V or greater from its offset value isdeclared to be a touch.

At the edges, typical offset values may be:

-   -   Untouched “offset” value=0.75V    -   Touched value=25 mV(example pulse level at sensor edge without        touch)*40%(fall in level due to touch)*50(amplifier        gain)−0.5V(diode drop)=0.25V    -   Difference=0.5V

With a difference threshold of 0.5V it would be very marginal whetherthis would be detected as a touch or not.

Therefore a “percentage scanner” is proposed. This adds an additionalprocessing stage, in which the voltage drop is calculated as apercentage of the untouched voltage level. This percentage value canthen be compared against a threshold percentage drop, and for use in theinterpolator.

Therefore, for a typical point in the middle of the sensor:

-   -   Untouched value=2.0V corresponding to a 50 mV pulse    -   Touched value=1.0V corresponding to a 30 mV pulse    -   Difference=1V corresponding to a 20 mV difference    -   Percentage change=20 mV/50 mV=40% drop.

And for a point at the edge of the same sensor:

-   -   Untouched value=0.75V corresponding to a 25 mV pulse    -   Touched value=0.25V corresponding to a 15 mV pulse    -   Difference=0.5V corresponding to a 10 mV difference    -   Percentage change=10 mV/25 mV=40% drop.

So if the threshold is set at, for example, a 20% drop, a touch can bedetected equally reliably in both the middle and edge of the sensor.Also the values provided to the interpolator are better than they wouldbe using the described alternatives, resulting in more accurate touchlocation estimates.

This method addresses the problem of non-uniform coupling across asensor, allowing consistent touch detection and accurate touch locationestimation with real sensors.

It will be understood that the particular component parts of which thevarious arrangements described above are comprised are in some exampleslogical designations. Accordingly, the functionality that thesecomponent parts provide may be manifested in ways that do not conformprecisely to the forms described above and shown in the diagrams. Forexample some aspects, particularly many aspects of the touch detectionmethods disclosed herein, may be implemented in the form of a computerprogram product comprising instructions (i.e. a computer program) thatmay be implemented on a processor, stored on a data sub-carrier such asa floppy disk, optical disk, hard disk, EPROM, RAM, flash memory or anycombination of these or other storage media, or transmitted via datasignals on a network such as an Ethernet, a wireless network, theInternet, or any combination of these of other networks, or realised inhardware as an ASIC (application specific integrated circuit) or an FPGA(field programmable gate array) or other configurable or bespoke circuitsuitable to use in adapting the conventional equivalent device.

It should be appreciated that the methods and apparatuses disclosedherein are complementary and some or all may be combined within a singlemethod or apparatus.

1. A touch sensitive display comprising a sensor having an input arrayof electrodes and, capacitively coupled thereto, an output array ofelectrodes; wherein said touch sensitive display comprises a controlleroperable to perform a scan operation at every intersection point of saidinput array, said scan operation comprising: measuring a touch value foran intersection point; determining a proportional difference betweensaid touch value and a base touch value for said intersection point as aproportion of said base touch value, wherein said base touch value isindicative of there being no touch event on the sensor; and comparingsaid proportional difference to a predetermined proportional touchthreshold so as to determine whether there is a touch event at thatpoint.
 2. The touch sensitive display as claimed in claim 1, whereinsaid controller is further operable to measure said base touch value foreach intersection point.
 3. The touch sensitive display as claimed inclaim 1, wherein said proportional touch threshold is defined as thedifference between a touch value and a base touch value, as a proportionof said base touch value, indicative of there being a touch event. 4.(canceled)
 5. The touch sensitive display as claimed in claim 1, whereinsaid predetermined proportional touch threshold is between 10% and 30%.6. The touch sensitive display as claimed in claim 1, wherein saidsensor is operable such that a change in voltage applied to one of theelectrodes of said input array results in a voltage pulse on the outputarray electrodes, the magnitude of which is indicative of a touch event.7. The touch sensitive display as claimed in claim 1, wherein each ofthe electrodes of said input array of electrodes and of said outputarray of electrodes comprises a conducting wire individually insulatedwith an insulating coating.
 8. The touch sensitive display as claimed inclaim 7, wherein the conducting wire is of diameter 8 μm to 18 μm. 9.The touch sensitive display as claimed in claim 7, wherein theinsulating coating comprises a polyurethane coating having a thicknessof 1 μm to 2 μm.
 10. The touch sensitive display as claimed in claim 1,wherein the electrodes of said input array of electrodes are arrangedsubstantially orthogonal to the electrodes of said output array ofelectrodes, each intersection point being where an input electrodecrosses an output electrode.
 11. The touch sensitive display as claimedin claim 1, wherein said input array of electrodes and said output arrayof electrodes are laid over each other so as to form a single electrodearray layer in the panel.
 12. The touch sensitive display as claimed inclaim 1, wherein said controller is further operable to detect a touchat an intersection point by transmitting a pulse on an input electrodeand monitoring a corresponding pulse energy on one or more outputelectrodes, said corresponding pulse arising due to capacitive couplingbetween the input electrode and the one or more output electrodes.
 13. Amethod of operating a touch sensitive display, said touch sensitivedisplay comprising a sensor having an input array of electrodes and,capacitively coupled thereto, an output array of electrodes; said methodcomprising performing a scan operation at every intersection point ofsaid input array, said scan operation comprising: measuring a touchvalue for an intersection point; determining a proportional differencebetween said touch value and a base touch value for said intersectionpoint as a proportion of said base touch value, wherein said base touchvalue is indicative of there being no touch event on the sensor; andcomparing said proportional difference to a predetermined proportionaltouch threshold so as to determine whether there is a touch event atthat point.
 14. The method as claimed in claim 13 further comprising thestep of measuring said base touch value for each intersection point. 15.The method as claimed in claim 14, wherein said proportional touchthreshold is defined as the difference between a touch value and a basetouch value, as a proportion of said base touch value, indicative ofthere being a touch event.
 16. (canceled)
 17. The method as claimed inclaim 13, wherein said predetermined proportional touch threshold isbetween 10% and 30%.
 18. The method as claimed in claim 13, wherein achange in voltage applied to one of the electrodes of said input arrayresults in a voltage pulse on the output array electrodes, the magnitudeof which is indicative of a touch event.
 19. The method as claimed inclaim 13, wherein the electrodes of said input array of electrodes arearranged substantially orthogonal to the electrodes of said output arrayof electrodes, each intersection point being where an input electrodecrosses an output electrode.
 20. The method as claimed in claim 13,wherein said input array of electrodes and said output array ofelectrodes are laid over each other so as to form a single electrodearray layer in the panel.
 21. The method as claimed in claim 13, whereinsaid method further comprises detecting a touch at an intersection pointby transmitting a pulse on an input electrode and monitoring acorresponding pulse energy on one or more output electrodes, saidcorresponding pulse arising due to capacitive coupling between the inputelectrode and the one or more output electrodes.
 22. A program carriercomprising computer readable instructions which, when run on a suitableapparatus, cause the apparatus to perform the method of claim
 13. 23.The method as claimed in claim 13, wherein said proportional touchthreshold is defined as the difference between a touch value and a basetouch value, as a proportion of said base touch value, indicative ofthere being a touch event.